1. Field of the Invention
The present invention relates to a transflector. A transflector is an optical element that when illuminated with light from one direction will reflect a significant proportion of the incident light, and that when illuminated with light from another direction will transmit a significant proportion of the incident light. The present invention also relates to a transflective display device, in particular to a transflective electro-optical display device, for example such as a transflective liquid crystal display device.
2. Description of The Related Art
Reflective display devices are well known. The principle of a reflective display device is illustrated in FIG. 1. This shows a reflective display device that consists of a light-modulating element 4 and a reflector 6 disposed behind the light-modulating element 4. The light-modulating element is formed of a layer 1 of electro-optic material, for example such as a liquid crystal material, disposed between a front substrate 2 and a rear substrate 3. The front substrate 2 is transparent, but the rear substrate 3 need not be transparent. (The term xe2x80x9cfrontxe2x80x9d of the display device refers to the side of the display device that is intended to face a viewer when the display is in use.) The display device shown in FIG. 1 is intended to be illuminated from the front by ambient light. A representative source of ambient light 5 is shown in FIG.1.
Light incident on the front of the light-modulating element 4 passes through the light-modulating element, is reflected by the reflector 6, and passes back through the light-modulating element to a viewer 7. (In practice, light from the light source 5 will undergo refraction as it passes through the optical modulating element, in particular at the front surface of the front substrate 2. This refraction is not shown in FIG. 1 for clarity of explanation.) The image seen by the viewer 7 can be changed by varying the voltage applied across the electro-optic layer. A reflective device has the advantage that, under suitable illumination conditions, it can utilise ambient light and does not require its own light source. As a result the power consumption of the display device is reduced, and this is advantageous for a battery-powered device.
For such a reflective display to operate effectively, it is necessary that sufficient of the ambient light incident on the display is directed towards the viewer so that a sufficiently bright display is produced. Blazed reflectors can be used to redirect ambient light impinging on a reflective display at an oblique angle so that, after reflection, it exits the display substantially at normal incidence. This is advantageous since viewers of a display generally view the display from the normal direction, or from a near-normal direction, and the use of a blazed reflector creates a higher reflectance of the display towards such a viewer.
Conventional reflective display devices are generally provided with a scattering reflector. A scattering reflector reflects incident light into a range of angles about the direction of specular reflection. This provides even illumination over the area of the display, and also prevents the viewer from seeing a reflected image of the ambient light source.
Co-pending UK patent application No 9909379.1 and corresponding Japanese patent application No 2000-123 267 disclose a scattering blazed reflector that eliminates diffraction effects.
EP 0 883 015 A1 discloses a scattering reflector for an LC display consisting of a reflector and a light diffusion layer disposed above the reflector. The scattering reflector is used to reflect ambient light back through the LC display. In this reflector, the entire surface of the scattering reflector is covered by a metallic reflective layer that is not transmissive.
In order to improve visibility of images, ambient light is selectively reflected from the scattering reflector into a direction, which is different from the direction of reflection of ambient light from front surfaces of the display. The reflected light intensity therefore depends on the angle under which the reflected light intensity is observed. In order to broaden the range of angles into which light is reflected, a light diffusion layer is provided on the reflector.
The magnitude of the overall reflectivity of this reflector, however, does not depend on the angle at which light is incident on the surface of the reflector.
in practice, the optical modulating element 4 of the display device of FIG. 1 will require other components to display an image, for example such as electrodes to allow the electro-optic layer 1 to be addressed: the optical modulating element may also comprise one or more polarising or retarding elements. These components have been omitted from FIG. 1 for clarity.
Reflective display devices have the disadvantage that they cannot operate in conditions of low ambient light, since insufficient light is reflected to form a satisfactory image. Where a display device is required to operate in conditions of low ambient light it is, therefore, necessary for the display device to be provided with an auxiliary light source that can illuminate the light-modulating element in conditions of low ambient light. The auxiliary light source can be disposed either in front of or behind the light-modulating element. A reflective display device in which the auxiliary light source is disposed in front of the light-modulating element is not relevant to the present invention, and will not be described further.
FIG. 2 illustrate the principle of operation of a reflective-type display device in which an auxiliary light source 9 is disposed on the opposite side of the light-modulating element 4 to the viewer 7. Since the auxiliary light source 9 is disposed behind the light-modulating element, it is generally known as a xe2x80x9cback-lightxe2x80x9d. Moreover, since the auxiliary light source 9 is disposed behind the light-modulating element, the rear substrate 3 is required to be transparent.
In place of the reflector 6 of the display device of FIG. 1, the display device of FIG. 2 is provided with a transflector 8 disposed behind the optical modulating element 4, between the optical modulating element 4 and the auxiliary light source 9. As noted above, a transflector is an optical element that when illuminated with light from one direction will reflect a significant proportion of the incident light, and that when illuminated with light from another direction will transmit a significant proportion of the incident light. (The terms xe2x80x9ctransmitxe2x80x9d and xe2x80x9creflectxe2x80x9d relate to the intended wavelength range of light with which the device will be used.) The display device of FIG. 2 is thus known as a transflective display device, since it contains a transflector.
FIG. 2 shows the operation of the transflective display device. When ambient light is incident on the optical modulating element 4, it passes through the optical modulating element and is reflected by the transflector a back to the viewer 7 (as is the case in FIG. 1, the effects of refraction have been ignored in FIG. 2). Thus, operation in ambient light is the same as for a reflective display device, and the back-light 9 does not need to be illuminated.
In conditions of low ambient light the back-light 9 is illuminated, and light from the back-light 9 passes through the transflector 8, through the optical modulating element 4, and reaches the viewer 7. In this mode of operation the transflector simply serves to transmit light, and it is preferable that the transflector has as great a transmissivity as possible to produce a bright image. In contrast, when the display device is illuminated by the ambient light source 5 the tranoflector 8 acts as a reflector, and it is desirable that the transflector reflects as much as possible of the light from the ambient light source 5 back to the viewer 7. Thus, in order to obtain a bright image in all lighting conditions, the transflector 8 should have a high reflectivity to light incident on its front face and a high transmissivity to light incident on its back face. In conventional transflectors, however, these two properties are in conflict with one another.
A transflective display device of the general type shown in FIG. 2 is generally provided with a scattering transflector. This is. to ensure that the optical modulating element is evenly illuminated over its entire display area, and to prevent the viewer from seeing an image of the ambient light source 5 or the back-light 9. A display device of the general type shown in FIG. 2 in which a scattering transflector is disposed behind a liquid crystal cell to disclosed in U.S. Pat. No. 4,106,859. It is also known to dispose the scattering. transflector inside the liquid crystal cell, rather than outside and behind the liquid crystal cell, and such a device is disclosed in U.S. Pat. No. 4,648,691.
One prior art scattering transflector is disclosed in U.S. Pat. No. 4,268,127 and U.S. Pat. No. 4,298,249. This prior art scattering transflector consists essentially of a continuous, partially-reflective, partially-transparent layer disposed over the entire surface of a transparent substrate. Such a scattering transflector is manufactured by, for example, disposing a thin metallic film onto a substrate that has an undulating upper surface. The thickness of the metallic film is chosen such that every point of the film has a non-zero reflectivity and a non-zero transmissivity. This prior art scattering transflector has the disadvantage that there is a xe2x80x9ctrade-offxe2x80x9d between the reflectivity of the transflector and the transmissivity of the transflector. If the metallic film is made thicker the reflectivity is increased, but the transmissivity is reduced.
A second prior art scattering transflector is disclosed in U.S. Pat. No. 4,040,727. In this scattering transflector, a thick metallic layer is disposed over parts of a substrate that has an undulating surface Other parts of the surface of the substrate are not provided with the metallic film. The thickness of the metallic film is chosen so that it is nominally totally reflective (in practice, of course, some light will be absorbed by the film). The portions of the transflector where the metallic film is deposited are thus nominally totally reflective and non-transmissive. However, the portions where the metallic film is not disposed are nominally completely transmissive and non-reflective. This prior art scattering transflector again has the problem that there is a xe2x80x9ctrade-offxe2x80x9d between the overall reflectivity and the overall transmissivity of the tranoflector. For example, if the area of the transflector that is covered by the metallic film is increased the overall reflectivity of the transflector is increased., but the overall transmissivity of the transflector is decreased.
A first aspect of the present invention provides scattering transflector wherein the reflectivity or the magnitude of the overall reflectivity of a surface of the transflector is dependent on the angle at which light is incident on the surface of the transflector. It is thus possible for the transflector to have a high reflectivity for light incident on the transflector from one direction, for example an oblique direction, while having a high transmissivity for light incident in another direction, for example a direction generally perpendicular to the transflector.
A second aspect of the present invention provides a scattering transflector comprising portions of reflective material and transmissive portions, wherein the ratio between the reflective area and the transmissive area is dependent on the direction of incidence of light onto the :transflector. Accordingly, depending on the angle of incidence of light onto the surface of the transflector. not the entire apparent surface of the transflector is covered by reflective material. It is thus possible to obtain a high reflectivity for light incident on the transflector from one direction, for example an oblique direction, while having a high transmissivity for light incident in another direction, for example a direction generally perpendicular to the transflector
The transflector may comprise: a substrate having an undulating surface, a plurality of first portions of the undulating surface being inclined in a first direction and a plurality of second portions of the undulating surface being inclined in a second direction different from the first direction; and a reflective material disposed over selected parts of the undulating surface of the substrate; wherein the reflective material is disposed over a greater area of first portions of the undulating surface than second portions of the undulating surface. The apparent proportion of the area of the transflector that is coated with the reflective material will vary depending on the angle from which the transflector is viewed from, and will appear to be greatest when the transflector is viewed from a direction that is perpendicular to the first portions of the surface. Because the apparent proportion of the transflector that is covered with the reflective layer varies with angle, it is possible for a transflector of the present invention to have a greater reflectivity and a greater transmissivity than a conventional transflector, provided that the transflector is used such that the illumination angle when it to used in a reflective mode is different from the illumination angle when it is used in a transmissive mode. This can be done by, for example, illuminating the display device in an off-axis direction in the reflective mode and illuminating the display device in a substantially on-axis direction in the transmissive mode.
In a transflector of the invention light is transmitted through portions of the transflector where the reflective material is not provided, so there is no need for light to be transmitted through the reflective material. The reflective material can therefore be made as highly reflective as possible.
Moreover, in a prior art transflector of the type in which a continuous part-reflective, part-transmissive layer is disposed over the entire surface of the substrate there will be an absorption loss when the transflector is operated in a transmissive mode, since some light will inevitably be absorbed by the part-reflective, part-transmissive layer. Such an absorption loss does not occur in a transflector of the present invention, since light is transmitted through portions of the transflector where the reflective material is not provided.
The reflective material may be disposed over substantially all the first portions of the undulating surface of the substrate, and it may be disposed over all the first portions of the undulating surface of the substrate. The reflective material may be disposed over substantially none of the second portions of the undulating surface of the substrate, and it may be disposed over none of the second portions of the undulating surface of the substrate. Thus, preferably not all portions of the substrate are covered by the reflective material.
The average angle of inclination of each first portion of the undulating surface of the substrate may be equal in magnitude and opposite in sign to the average angle of inclination of each second portion of the undulating surface of the substrate. This provides a symmetric scattering reflector. Alternatively, the average angle of inclination of each first portion of the undulating surface of the substrate may be different in magnitude and opposite in sign to the average angle of inclination of each second portion of the undulating surface of the substrate. This provides an asymmetric scattering reflector.
The reflective material may be a metallic material.
The substrate of the transflector may be a polymeric substrate, or it may be d glass substrate.
Two or more of the portions of the reflective material may be electrically connected to one another. The transflector may further comprise a transparent, electrically conductive layer disposed over the undulating surface of the substrate. The portions of reflective material may be disposed over the transparent, electrically conductive layer. The two or more of the portions of the reflective material layer may be electrically connected to one another by the transparent, electrically conductive layer.
A portion of the substrate may have a thickness of substantially zero.
A third aspect of the present invention provides a display device comprising: an optical modulating element and a scattering transflector.
The transflector may be disposed relative to the optical modulating element such that angle between the normal to each first portion and the direction in which light is incident on the display surface of the display device is less than the angle between the normal to the display surface of the display device and the direction in which light is incident on the display surface.
The transflector may be disposed behind the optical modulating element, or it may be disposed within the optical modulating element.
The optical modulating element may comprise an electro-optic layer disposed between a first substrate and a second substrate. The electro-optic layer may be a liquid crystal layer.
The substrate of the transflector may be one of the substrates of the optical modulating element.
According to one aspect of the present invention, there is provided a scattering transflector. The reflectivity of a surface of the scattering transflector is dependent on the angle at which light is incident on the surface of the transflector.
According to an embodiment of the invention, the scattering transflector may include: a substrate having an undulating surface, a plurality of first portions of the undulating surface being inclined in a first direction and a plurality of second portions of the undulating surface being inclined in a second direction different from the first direction: and a reflective material disposed over selected parts of the undulating surface of the substrate. The reflective material may be disposed over a greater area of first portions of the undulating surface than second portions of the undulating surface.
According to an embodiment of the invention, the reflective material may be disposed over substantially all the first portions of the undulating surface of the substrata.
According to an embodiment of the invention, the reflective material may be disposed over all the first portions of the undulating surface of the substrate.
According to an embodiment of the invention, the reflective material may be disposed over substantially none of the second portions of the undulating surface of the substrate.
According to an embodiment of the invention, the reflective material may be disposed over none of the second portions of the undulating surface of the substrate.
According to an embodiment of the invention, the average angle of inclination of each first portion of the undulating surface of the substrate may be equal in magnitude and opposite in sign to the average angle of inclination of each second portion of the undulating surface of the substrate.
According to an embodiment of the invention, the average angle of inclination of each first portion of the undulating surface of the substrate may be different in magnitude and opposite in sign to the average angle of inclination of each second portion of the undulating surface of the substrate.
According to an embodiment of the invention, the reflective material may be a metallic material.
According to an embodiment of the invention, the substrate of the transflector may be selected from a group consisting of a polymer substrate and a glass substrate.
According to an embodiment of the invention, two or more of the portions of the reflective material may be electrically connected to one another.
According to an embodiment of the invention, the scattering transflector may further include a transparent, electrically conductive layer disposed over the undulating surface of the substrate.
According to an embodiment of the invention, the portions of reflective material may be disposed over the transparent, electrically conductive layer.
According to an embodiment of the invention, two or more of the portions of the reflective material layer may be electrically connected to one another by the transparent, electrically conductive layer.
According to an embodiment of the invention, the two or more of the portions of the reflective material layer may be electrically connected to one another by the transparent, electrically conductive layer.
According to an embodiment of the invention, a portion of the substrate may have a thickness of substantially zero.
According to an embodiment of the invention, either the first or second portions of the undulating surface of the substrate may not be flat.
According to an embodiment of the invention, the first portions of the undulating surface of the substrate may have a concave surface in section.
According to an embodiment of the invention, the first portions of the undulating surface of the substrate may have a concave surface in section and the second portions of the undulating surface of the substrate may have a convex surface in section.
According to an embodiment of the invention, the reflective material may be disposed in strips along the first portions of the undulating surface of the substrate.
According to an embodiment of the invention, the strips of reflective material disposed along the first portions of the undulating surface of the substrate may have some degree of randomness, when seen in plan view.
According to an embodiment of the invention, at least one edge of each strip of reflective material may be irregular.
According to an embodiment of the invention, the reflective material may be disposed randomly on the first portions of the undulating surface of the substrate in discrete portions.
According to an embodiment of the invention, the surf ace of the transflector may include a plurality of discrete raised portions.
According to an embodiment of the invention, the reflective material may be disposed over part of at least some of the discrete raised portions.
According to an embodiment of the invention, the locations of the discrete raised portions may be randomly distributed.
According to another aspect of the present invention, there is provided a display device including an optical modulating element and a scattering transflector. The reflectivity of a surface of the transflector is dependent on the angle at which light is incident on the surface of the transflector.
According to an embodiment of the invention, the transflector may further include: a substrate having an undulating surface, a plurality of first portions of the undulating surface being inclined in a first direction and a plurality of second portions of the undulating surface being inclined in a second direction different from the first direction; and a reflective material disposed over selected parts of the undulating surface of the substrate. The reflective material may be disposed over a greater area of first portions of the undulating surface than second portions of the undulating surface.
According to an embodiment of the invention, in the transflector, the reflective material may be disposed over substantially all the first portions of the undulating surface of the substrate.
According to an embodiment of the invention, in the transflector, the reflective material may be disposed over all the first portions of the undulating surface of the substrate.
According to an embodiment of the invention, in the transflector, the reflective material may be disposed over substantially none of the second portions of the undulating surface of the substrate.
According to an embodiment of the invention, in the transflector, the reflective material may be disposed over none of the second portions of the undulating surface of the substrate.
According to an embodiment of the invention, in the transflector, the average angle of inclination of each first portion of the undulating surface of the substrate may be equal in magnitude and opposite in sign to the average angle of inclination of each second portion of the undulating surface of the substrate.
According to an embodiment of the invention, in the transflector, the average angle of inclination of each first portion of the undulating surface of the substrate may be different in magnitude and opposite in sign to the average angle of inclination of each second portion of the undulating surface of the substrate.
According to an embodiment of the invention, the reflective material of the transflector may be a metallic material.
According to an embodiment of the invention, the substrate of the transflector may be selected from a group consisting of a polymer substrate and a glass substrate.
According to an embodiment of the invention, two or more of the portions of the reflective material may be electrically connected to one another.
According to an embodiment of the invention, the display device may further include a transparent, electrically conductive layer disposed over the undulating surface of the substrate.
According to an embodiment of the invention, the portions of reflective material may be disposed over the transparent, electrically conductive layer.
According to an embodiment of the invention, two or more of the portions of the reflective material layer may be electrically connected to one another by the transparent, electrically conductive layer.
According to an embodiment of the invention, a portion of the substrate may have a thickness of substantially zero.
According to an embodiment of the invention, either the first or second portions of the undulating surface of the substrate may not be flat.
According to an embodiment of the invention, the first portions of the undulating surface of the substrate may have a concave surface in section.
According to an embodiment of the invention, the first portions of the undulating surface of the substrate of the substrate may have a concave surface in section and the second portions of the undulating surface of the substrate may have a convex surface in section.
According to an embodiment of the invention, the reflective material may be disposed in strips along the first portions of the undulating surface of the substrate.
According to an embodiment of the invention, the strips of reflective material disposed along the first portions of the undulating surface of the substrate may have some degree of randomness, when seen in plan view.
According to an embodiment of the invention, at least one edge of each strip of reflective material may be irregular.
According to an embodiment of the invention, the reflective material may be disposed randomly on the first portions of the undulating surface of the substrate in discrete portions.
According to an embodiment of the invention, the surface of the transflector may include a plurality of discrete raised portions.
According to an embodiment of the invention, the reflective material may be disposed over part of at least some of the discrete raised portions.
According to an embodiment of the invention, the locations of the discrete raised portions may be randomly distributed.
According to an embodiment of the invention, at least one portion of the substrate may have a thickness of substantially zero for forming at least one transmissive window.
According to an embodiment of the, invention, the reflective material may not be provided in the at least one transmissive window of the substrate.
According to an embodiment of the invention, the optical modulating element may include an electro-optic layer of a thickness so as to form a half-wavelength retarder in the at least one transmissive window of the substrate and a quarter-wavelength retarder elsewhere.
According to an embodiment of the invention, the transflector may be disposed relative to the optical modulating element such that an angle between the normal to each first portion and the direction in which light is incident on the display surface of the display device is less than the angle between the normal to the display surface of the display device and the direction in which light is incident on the display surface.
According to an embodiment of the invention. the transflector may be disposed behind the optical modulating element.
According to an embodiment of the invention, the transflector may be disposed within the optical modulating element.
According to an embodiment of the invention, the optical modulating element may include an electro-optic layer disposed between a first substrate and a second substrate.
According to an embodiment of the invention, the electro-optic layer may be a liquid crystal layer.
According to an embodiment of the invention, the substrate of the transflector may be one of the substrates of the optical modulating element.
According to still another aspect of the present invention, there is provided a scattering transflector including portions of reflective material and transmissive portions. The ratio between the reflective area and the transmissive area depends on the direction of incidence of light.
According to still another aspect of the present invention, there is provided a display device including an optical modulating element and, a scattering transflector including portions of reflective material and transmissive portions. The ratio between the reflective area and the transmissive area of the scattering transflector depends on the direction of incidence of light.